The asparagine-linked oligosaccharides of mature glycoproteins have a wide variety of structures, most of which can be assigned to two general families: the high-mannose class and the complex class. Both types contain Man3GlcNAc2 as a central core; the high-mannose type contains additional mannose residues, and the complex type contains other sugars (galactose, fucose, and sialic acid). During the last year evidence has accumulated that both high-mannose and complex asparagin-linked oligosaccharides arise from a common precursor, identified as a lipid-linked oligosaccharide with the structure Man4-Man - Man-GlcNAc-GlcNAc-PP-lipid - Glc-Glc-Glc-Man-Man-Man. This species appears to be ubiquitous; it has been detected in 3T3 and Nil-8 cells, in chinese hamster ovary cells, in human diploid fibroblasts, and in normal, Sindbis virus-infected, and vesicular stomatitis virus-infected chick embryo cells. We plan to study in detail the synthesis and processing of this oligosacharide in normal and transformed cells.